1. Field of the Invention
The present invention relates to a method of manufacturing an electrode to be used in a battery, the method being achieved by applying a paste-like coating material to a metal foil as it is fed forward to form a coated foil, and drying the coated foil formed with a coating layer or film as the coated foil is fed forward to pass through an oven of a drying machine arranged in line.
2. Related Art
In recent years, secondary batteries such as lithium ion batteries are used as power sources of for example a hybrid car, an electric vehicle, a cellular phone, a portable device, etc. In the case of manufacturing lithium ion batteries, this manufacturing process includes a coating process in which a feeding device feeds a metal foil (a current collector) in a flat form from a raw foil roll and a coating device applies a coating material ejected from a die onto the metal foil supported on a backup roller to form a coated foil. In a next drying process, the coated foil is dried by passing through an oven of a drying machine installed in line on a feeding path of the coated foil. The coating material is a paste-like coating solution prepared in advance before the coating process in such a manner that a plurality of active materials, binder resin for binding those active materials to each other and binding the active materials to a current collector, and others are mixed in a solvent (water or an organic solvent). In the case where the electrode is a positive electrode, the coating material contains a positive active material and also a conductive material imparting electric conductivity to the positive active material. In the case where the electrode is a negative electrode, the coating material contains a negative active material.
Meanwhile, in the drying process, when the coated foil is rapidly dried in the oven of the drying machine, the solvent contained in the coating layer evaporates, leading to cracks in a front surface of the coating layer. Further, the binder resin moves toward the front surface and is solidified during drying, causing migration in the coating layer. In case cracks or migration occur in the coating layer, a bonding strength of the coating layer to the current collector decreases. In subsequent processes following the drying process, accordingly, the coating layer may peel from the current collector. Such a battery is unusable as a product in terms of quality control. Examples of the drying process arranged to avoid the above problems are disclosed in Patent Documents 1 and 2.
FIG. 9 is a diagram to explain a drying process in Patent Document 1. In this document, as shown in FIG. 9, a drying oven 170 is partitioned into three areas 172, 174 and 176 continuously arranged along a feeding path. After a coating process, the coating layer formed on a current collector sheet 120 is dried as it is fed forward by a feeding device to pass through the first area 172 on an entrance side, the second area 174, and the third area 176 on an exit side in this order. The coating layer is dried by being exposed first to hot air of 100° C. and 90% RH or more by a heater 192 and a humidifier 100 in combination in the first area 172, to hot air of 120° C. in the second area 174, and then finally to hot air of 150° C. in the third area 176.
In Patent Document 2, when a sheet material applied with a resin solution is to be dried, hot water vapor (saturated vapor of about 100° C.) is sprayed in advance on the surface of the sheet material. Thus, a drying speed of the resin solution is adjusted to be almost equal to a drying speed that causes no migration.